This invention relates generally to anchors and particularly to a drag embedment anchor effective in a wide range of seafloor materials, and especially for use in tandem to effectively develop a system capacity equal to or greater than twice the capacity of a single anchor.
The mooring anchor disclosed in U.S. Pat. No. 3,015,299 is a high efficiency anchor which, in the past, has satisfied some of the needs for a general purpose anchor, and has provided a baseline for better and more efficient anchors. However, the aforementioned anchor has experienced structural problems, has difficulty tripping and digging into hard soil, is not suitable for fabrication in sizes above 15,000 lbs.; further, it is not suitable for use in tandem. Nevertheless, the U.S. Pat. No. 3,015,299 anchor has been the best all around performer and the most efficient general soil anchor of all the prior art anchors. Although some of the newer high efficiency anchors are exceptional single soil type anchors, none perform fully satisfactory for good overall anchoring capability in a variety of situations where seafloor soil type is not known. Some anchors fail to trip and dig-in in soft mud; some have difficulty penetrating very dense sand; some are only marginally stable in sand and mud; and some have only moderate or marginal capacity in mud. And, none of the prior anchors when used in tandem developed much more than the full rated capacity of a single anchor, frequently less. Some became very unstable when used in tandem.
Tandem anchors offer a lighter weight alternative to single large anchors which are difficult to install and almost impossible to retrieve. Tandem anchors, also called piggyback anchors, are used in commercial practice, but only when the first anchor fails to hold its rated (single anchor) capacity. With a piggyback anchor attached, the two anchor system is loaded to the expected capacity of one single anchor. Conventional commercial anchors presently available are not designed to tolerate or hold more than the rated capacity of a single anchor, and when used in tandem, the combined capacity of the tandem system is expected to equal only the rated capacity of the single primary anchor.